1. Field of the Invention
The invention relates to an ion sensitive field effect transistor (ISFET), and in particular to a PbTiO3/SiO2-gated ISFET device fabricated through a sol-gel process to detect hydrogen ions in an aqueous solution. The invention also relates to a method for forming a sensing film for fabrication of a PbTiO3/SiO2-gated ISFET device.
2. Description of the Related Art
The ion sensitive field effect transistor (ISFET) is constructed by substituting a sensing film for the metal gate on the gate oxide of a traditional MOSFET. When the ISFET is dipped into a solution, the interfacial potential between the sensing film and the solution will influence the semiconductor surface since only an extremely thin dielectric (that is, the gate oxide) separates the sensing film and the semiconductor surface. This influences the charge density in the inversion layer of the semiconductor surface, and thereby modulates the channel current passing through the ISFET. Therefore, by utilizing this characteristic, the pH value or other ion concentration in the solution can be deduced from the measurement of source/drain current and the gate voltage of the ISFET. Furthermore, the potential difference on the interface between the sensing film and the solution is in relation to the ion activity in the solution. The hydrogen ion activity in the solution can be measured by using different channel currents caused by different interfacial potential differences in various solutions with different hydrogen ion activity.
The patents, which are related to the formation of the ISFET or the measurement of the ISFET, are listed hereinafter.
(1) U.S. Pat. No. 6,531,858 B2 issued to the same inventor, Chou, J. C. and Tsai, H. M., discloses a method of measuring the hysteresis value and the drift value of an a-Si:H ISFET.
(2) U.S. Pat. No. 6,573,741 B2 issued to the same inventor, Chou, J. C., and Wang, Y. F. discloses a method and an apparatus for measuring the temperature parameters of an ISFET that uses hydrogenated amorphous silicon as a sensing film. The method uses the measurements of the temperature parameters and the source/drain current and gate voltage in an unknown solution to sense the ion concentration and the pH value of the unknown solution.
(3) U.S. Pat. No. 5,387,328 issued to Sohn, B. K. discloses a method of measuring the glucose concentration by fixing the enzyme on a sensing film and using platinum (Pt) as a reference electrode. The sensor has a Pt electrode being capable of sensing all biological substances which generate H2O2 in enzyme reaction.
(4) U.S. Pat. No. 5,319,226 issued to Sohn, B. K. and Kwon, D. H. discloses a Ta2O5 sensing film deposited by a radio frequency sputtering method on an ISFET, wherein the ISFET consists of a Ta2O5/Si3N4/SiO2 structure. In this case, a Ta2O5 film with a thickness between 400 and 500 Å is formed on a Si3N4/SiO2 dielectric layer, and the Ta2O5 film outside the gate is removed by a lift-off process utilizing a positive PR film. Compared with the traditional pH-ISFET, this invention provides better sensitivity and reliability.
(5) U.S. Pat. No. 5,314,833 issued to Lee, K. H.; Cho, K. I.; and Lee, Y. T. discloses a method comprising steps of depositing a silicon film on a GaAs substrate and doping arsenic/phosphorous ions into the silicon film to fabricate the gate with lower resistance. This can reduce the thermal effect on the device, and improve operational characteristics.
(6) U.S. Pat. No. 6,617,190 B2 issued to the same inventor, Chou, J. C., and Chiang, J. L. discloses an ISFET comprising an H+-sensing membrane consisting of RF-sputtering a-WO3. The a-WO3/SiO2-gated ISFET is highly sensitive in aqueous solutions, and particularly in acidic aqueous solutions, having sensitivity ranging from 50 to 58 mV/pH and high linearity. Accordingly, the ISFET can be also used to detect an effluent.
(7) U.S. Pat. No. 5,061,976 issued to Shimomura T.; Yamaguchi S.; Suzuki T.; and Oyama N. discloses a method that covers a carbon thin film on the gate oxide of the ISFET and then covers a 2, 6-xylenol electrolytically polymerized film thereon. The ISFET has the ability to sense hydrogen ions and the advantages of short drift time, high reliability and insensitivity to light. When particular thin films cover the ISFET, particular kinds of ions can be detected.
(8) U.S. Pat. No. 4,660,063 issued to Anthony, T. R. discloses a method of performing both laser drilling and solid diffusion to form a three-D diode array on a semiconductor wafer. The laser is first employed to drill the wafer, and then the impurities are diffused from the hole to form a cylindrical PN junction, forming a non-plane ISFET structure.
(9) U.S. Pat. No. 4,735,702 issued to Anthony, T. R. provides a polymer covered on an oxide layer of ISFET, wherein a chemical bond is formed on the interface between the polymer and the oxide layer to form a sensitive film.
(10) U.S. Pat. No. 5,911,873 issued to McCarron, R. T. and Gray, J. R. discloses a device that comprises ISFET, a reference electrode device, ISFET control circuits, memory, measuring circuits and diagnostic circuits to measure ion concentration in the solution. The ISFET control circuits operate the equipment at a certain drain/source voltage and a gate/source voltage that is relative to n successive drain currents. The memory stores the repetitions of the ISFET characteristics, the n successive drain currents, and gate/source voltage. The measuring circuits measure ion concentration by a group of the n successive drain currents and a gate/source voltage and the repetition of the ISFET characteristics. The diagnostic circuits measure the ISFET characteristics by using the n successive drain currents and gate/source voltage.
An ISFET using a PbTiO3 layer as a sensing film to detect H+ ions and the fabrication of the PbTiO3 sensing film by a Sol-Gel process have not been disclosed.